1. Field of the Invention
The present invention relates to an ATM cell assembly and disassembly device for assembling and disassembling the ATM cells carrying the data used in the ATM communication system.
2. Description of the Background Art
In recent years, in order to deal with the demands for variety of communications such as image communication and high speed data communication, the construction of the B-ISDN (Broadband-Integrated Service Digital Network) is in progress as an integrated communication network for facilitating efficient and flexible communication services. In a realization of the B-ISDN, the ATM (Asynchronous Transfer Mode) data exchange scheme is expected to play a key role. The ATM data exchange scheme is a scheme in which the data are loaded into a packet of a fixed length called ATM cell regardless of their attributes, and the communication service is realized by using such an ATM cell as a unit of data exchange.
In a conventional communication system, the packet assembly scheme has been applicable only to a kind of communication data which requires no real-time processing such as those for the usual data communication, so that a new packet assembly scheme is needed for the communication of the real-time data such as speech data and image data.
As such an ATM cell assembly scheme for the real-time data, there are propositions of: (a) a scheme for assembling an ATM cell by accumulating the successively entered real-time input data in a buffer, and attaching an ATM header to the accumulated data when a total amount of input data accumulated in the buffer reaches a predetermined amount such as that corresponding to a length of a payload section in each ATM cell to be assembled or a partially filling number for a payload section in each ATM cell to be assembled; and (b) a scheme in which a ratio of a speed of the input data and a transmission speed on the ATM side is determined in advance, and a cycle for outputting the ATM cell assembled from the input data and a cycle for outputting other cells such as empty cells are sequentially controlled to satisfy the predetermined transmission speed on the ATM side.
However, in scheme (a), the ATM cell generation cycle is going to be synchronous with the cycle of input data, so that it becomes necessary to provide a speed matching buffer for matching the transmission speed of the generated ATM cells with that suitable for the ATM side interface on the downstream side of the ATM cell assembly device, but this requirement leads to a considerable increase of the amount of hardware required for the communication system.
On the other hand, in scheme (b), it is impossible to provide a flexibility in the set up of the speed of the input data, so that a type of the input data that can be handled in the communication system is going to be restricted in view of the speed of the input data.
Now, one of the characterizing feature of the ATM communication scheme is the improvement of the network utilization efficiency based on the statistical multiplexing effect due to the label multiplexing of the cells on the transmission path. To this end, the ATM communication scheme requires a resource management which is not required in the conventional communication scheme.
More specifically, in order to share the resource in the network by the label multiplexing, each terminal which initiates the communication by making a call set up request reports parameters such as the peak rate and the average rate of the requested call as the UPC (Usage Parameter Control) parameters to the network. Then, the network carries out the resource management by making the judgement as to whether it is possible to guarantee the quality of service required to the network when the requested call is admitted or not according to the predetermined evaluation function, and then admitting the requested call only when it is judged that the required quality of service can be guaranteed even when the requested call is admitted. The terminal for which the requested call has been admitted in this manner should output the cells for the communication by observing the UPC parameters reported at a time of the call set up request, in order to prevent the outputted cells from being discarded by the policing function adopted in the network side.
This observance of the UPC parameters on the terminal side is usually achieved by modifying the cell traffic to be transmitted into a modified traffic satisfying the UPC parameters, and then outputting this modified traffic to the transmission path. This operation on the terminal side is called traffic shaping.
This traffic shaping has conventionally been achieved by a configuration shown in FIG. 1 in which the ATM cell flow generated at the ATM encoding and cell generation unit 501 is stored in a shaping buffer 502 once, and the ATM cells are taken out from this shaping buffer 502 under the control of a shaping controller 503 in accordance with the UPC parameters determined at a time of the call set up, and outputted to the transmission path. As the algorithm to be used by the shaping controller 503 in realizing such a traffic shaping, the leaky bucket algorithm and the sliding window algorithm are conventionally known.
In this manner, in the conventional communication scheme, the communication is realized in a form observing the UPC parameters by applying the traffic shaping on the cell flow outputted from the terminal before transmitting to the transmission path. However, in this conventional communication scheme, there is a possibility for the cells to be discarded at the shaping buffer 502. Namely, in the case the ATM encoding and cell generation unit 501 continually carries out the ATM encoding at a rate close to the peak speed, because the shaping buffer 502 can output the cells only at a constant rate, the amount of cells entered from the ATM encoding and cell generation unit 501 to the shaping buffer 502 may exceed the amount of cells outputted from the shaping buffer 502 to the transmission path, such that the overflow of the entered cells may occur at the shaping buffer 502.
As a solution to this problem of the shaping buffer overflow, the increasing of the capacity of the shaping buffer itself gives rise to another concern regarding the increase of the amount of hardware.
Also, in the ATM communication scheme, the communication service is expected to be charged in proportion to the peak rate used in the communication, so that the user will normally desire to suppress the peak rate to be used in the communication as low as possible in order to minimize the communication cost. In order to realize such a communication with the suppressed peak rate, the call set up request to the network should be made with a low peak rate and a peak rate value in the traffic shaping operation should be set to be low during the communication. Here, it possible for the peak rate used in the ATM encoding and cell generation unit 501 to exceed the peak rate used in the shaping buffer 502, and in such a case, depending on the encoding speed at the ATM encoding and cell generation unit 501, the shaping buffer 502 needs to accumulate a large number of cells, so that the large capacity is required for the shaping buffer 502, but this requirement also leads to the concern regarding the increase of the amount of hardware.
On the other hand, in the ITU-T (International Telecommnications Union-Telecommnication sector; formerly CCITT (The Consultative Committee for International Telegraph and Telephone)), the following schemes for preventing the deterioration of the congestion of the network are considered.
(a) The network attaches a flag for notifying the occurrence of the congestion to the user cell at the congested point in the network, the receiver side user receiving this user cell with this flag attached requests the transmitter side user to suppress the amount of transmission in response to this flag, and the transmitter side user suppresses the amount of transmission in accordance with this request from the receiver side user. This scheme is called the congestion control using FECN (Forward Explicit Congestion Notification).
(b) The network transmits a congestion notification cell from the congested point in the network to the transmitter side user, to request the suppression of the amount of transmission, and the transmitter side user suppresses the amount of transmission in response to this congestion notification cell. This scheme is called the congestion control using BECN (Backward Explicit Congestion Notification).
(c) The receiver side user recognizes the occurrence of the congestion of the network by detecting the cell loss due to the congestion, and requests the transmitter side user to suppress the amount of transmission, and the transmitter side user suppresses the amount of transmission in accordance with this request.
In any of these schemes, the transmitter side user to which the suppression of the amount of transmission is requested is expected to suppress the amount of transmission by controlling the shaping controller 503 to lower the parameters such as the peak rate and the average rate of the transmission traffic. However, in this case, the encoding speed at the ATM encoding and cell generation unit 501 is also required to be lowered in conjunction with the change of the parameters at the shaping controller 503, because otherwise the cells entering from the ATM encoding and cell generation unit 501 can exceed the cells outputted from the shaping buffer 502 to cause the shaping buffer overflow. Yet, such a changing of the parameters on both of the shaping controller 503 and the ATM encoding and cell generation unit 501 is going to be complicated as well as relatively slow because it calls for the parameter changing operations to be made at more than one parts in the configuration of FIG. 1.
Now, on the other hand, the ATM cell disassembly device is a device for recovering the data from the ATM cell obtained by the ATM cell assembly device. In this ATM cell disassembly device, in order to deal with the cell loss in the ATM communication network and the jitter in the cell arrival intervals called CDV (Cell Delay Variation), it is indispensable to provide a mechanism for absorbing and compensating these loss and CDV.
As a conventional scheme for the ATM cell disassembly for the real-time data, there is a scheme using a configuration shown in FIG. 2 in which the ATM cells received at an ATM processing unit 901 are pooled in a large capacity jitter absorption buffer 902 once, and the pooled ATM cells are taken out from this jitter absorption buffer 902 to an STM (Synchronous Transfer Mode) processing unit 903 at regular intervals, so as to absorb the jitter in the cell arrival intervals. Here, in a case the cell loss or the erroneous cell transmission is involved, the interpolation using dummy cell or dummy cell data and the discarding of the erroneously transmitted cell are carried out at either input or output side of the jitter absorption buffer 902.
Then, either at the output side of the jitter absorption buffer 902 or at the jitter absorption buffer 902 itself, the transfer to the receiver side clock is made, the necessary data obtained by removing unnecessary sections such as the header from the ATM cell are transmitted to the receiver side.
Here, in a case the operation frequency of the receiver side is predetermined, the ATM cell disassembly can be achieved by setting the timing to take out the cell from the jitter absorption buffer 902 to be equal to this operation frequency of the receiver side. However, in this scheme, the speed of the data outputted from the ATM cell disassembly device is predetermined and it is impossible to flexibly realize the various different speeds for the output data, or to flexibly deal with a case in which the receiver side comprises STM time slots whose number can vary in time.
Thus, the conventional ATM cell disassembly scheme lacks the flexibility as it fixes the ratio of the data input and output speeds to a predetermined ratio.